## The Diffraction Process of Colour Photography - Prof. R.W. Wood, 1900

The iridescence found in soap bubbles, crow's feathers, CD's, oil slicks, butterfly wings and countless other places is created by diffraction.

Diffraction is nature's digital; in much the same way that a series of 1's and 0's make up digital information, diffraction relies on a sort of ON and OFF system to create any colour of the spectrum.

In classical physics, the diffraction phenomenon is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings.

If said "small obstacles" & "small openings" are arranged in a constant & repeating manner, only certain waves will "make it out alive" upon passing through a diffraction grating, thus we will see a specific, predominating colour.

This of course is the modus operandi of the Lippmann color photograph. But there is another method in which to produce colour photographs by diffraction, invented by Professor R.W. Wood of the University of Wisconsin. The most complete account of this procedure can be found in A Handbook of Photography in Colours by Thomas Bolas, Alexander A. K. Tallent, Edgar Senior; published in 1900 by Marion & Co.

It begins on page 290; luckily it is on Google books -> http://books.google.com/books?id=1J0...page&q&f=false

In short; you have 3 ruled line screens, or gratings printed on transparent film. The gratings have lines that are 2000, 2400 and 2750 to the inch. These are the "masters" to create the diffraction gratings.

The novel part of the process is in these screens, which will be printed onto dichromated gelatin (DCG). With these fine lines impressed into the gelatin, a diffraction grating is created; meaning that light will go thru it, scatter, and depending on the viewing angle you will see different colours. Each of the 3 screens is capable of creating all colours, but not from the same angle of view. By distributing the lines differently in each screen, red, green & blue are created simultaneously from each respective screen if viewed from a specific angle.

As usual with 3 color work, you have 3 separation negatives of the subject, taken thru the standard separation filters. The negatives must be made into positives however, and each one is then placed in contact with the appropriate line screen, red w/ 2000 lpi, green w/ 2400 lpi and blue w/ 2750 lpi. These are then contact printed onto unpigmented dichromated gelatin.

Just as in the gum or carbon process, the DCG is then developed by etching with hot water to dissolve the unhardened gel. You are now left with 3 clear images. Due to the extremely fine lines courtesy of the line screens, these DCG plates are now diffraction gratings with diffraction "zones" relating to the separation positives which they were printed in tandem with.

Imagine a b&w separation positive of a red apple on a black background. The apple will be very transparent, surrounded by heavy density. Therefore, the clear portion will let thru the line screen and a diffraction grating corresponding to red will be made in relation to the apple.

This is the basis of the process. The only trick being that it requires a simple viewing apparatus; though I think that it basically just limits your viewing angle to a small degree, which could be achieved with a steady hand, or at most a piece of carboard with a hole in it.

The result is pure, permanent colour photographs!

I've attached a Word DOC with all figures and text from the book. I did the OCR from the Google books version.

Enjoy!